Researches and developments, in which a resistance change element, whose resistance value is changed depending on its physical state, is built into a semiconductor integrated circuit to expand its functions or performance, are now going on actively.
An MRAM (Magnetic Random Access Memory), for example, retains one-bit information by exploiting the property of a magnetic resistance element that its resistance is varied depending on its state of magnetization. On the other hand, a PRAM (Phase change RAM) retains one-bit information by exploiting the property of a phase change element of, for example, a chalcogenide alloy, that its resistance is varied depending on its crystal state. An RRAM (Resistive RAM) uses a metal oxide resistance change element of, for example, perovskite oxide, or a solid electrolyte resistance change clement. The RRAM exploits the property that the resistance of the resistance change elements is changed by controlling the voltage or current applied to these resistance change elements.
In the metal oxide resistance change element or in the solid electrolyte resistance change element, the ratio between the on-resistance and the off-resistance thereof is significant, varying by several orders of magnitude, as an example. Therefore, these resistance change elements may be applied not only for use in non-volatile memories, but also for freely programming truth values of logic gates to freely make or break interconnections. That is, the resistance change element may be caused to operate as a switching element for application to a non-volatile reconfigurable logic circuit (programmable logic Large Scale Integration).
To diversify the functions of the reconfigurable logic circuit to promote its implementation on electronic equipment, it is necessary to reduce the size of a switch interconnecting logic cells as well as to reduce its on-resistance. With a switch exploiting an electro-chemical reaction by solid electrolytes, it is possible to reduce the size in comparison with a CMOS (Complementary Metal-Oxide-Semiconductor) switch and, at the same time, to reduce the on-resistance by approximately one order of magnitude to, for example, ca. 100Ω, in comparison with the CMOS switch.
FIG. 10(a) to FIG. 10(d) illustrate a programming operation for a resistance change element that exploits the solid electrolyte. Referring to FIG. 10(a), a resistance change element, exploiting the solid electrolyte, is composed of a first electrode, supplying metal ions, a second electrode, not supplying metal ions, and an ion conduction layer sandwiched in-between.
Referring to FIG. 10(b), if, in applying a voltage across the two electrodes, the first electrode is set to a potential higher than the second electrode, metal is oxidized on the surface of the first electrode to metal ions which are delivered into the ion conduction layer. On the surface of the second electrode, on the other hand, the metal ions in the ion conduction layer are reduced to form metal which is precipitated.
Referring to FIG. 10(c), if a metal crosslinking reaching the surface of the second electrode from the surface of the first electrode is formed by metal precipitated, the state of conduction is changed from ionic conduction in the ion conduction layer to conduction passing through the metal crosslinking. In such case, the resistance between the two electrodes is rapidly decreased. That is, the switch shifts to an on-state (state of low resistance).
The programming operation from the off-state (high-resistance state) to the on-state in FIG. 10(b) is termed a “set operation.”
Referring to FIG. 10(d), if, in the state where metal crosslinking has been formed, the first electrode is set to a potential lower than that at the second electrode, metal is oxidized to form metal ions on the metal crosslinking surface where the potential is equal to that on the surface of the second electrode. The so formed metal ions are delivered into the ion conduction layer. On the surface of the first electrode, on the other hand, the metal ions in the ion conduction layer are reduced to form metal which is precipitated. Hence, the metal crosslinking gradually becomes thinner in thickness until, at a certain time point, the path of conduction between the electrodes via the metal crosslinking is disconnected. The electrical conduction between the two electrodes is changed at this time point from the conduction state via the metal crosslinking to the ion conduction via the ion conduction layer. Hence, the resistance between the two electrodes rises abruptly. That is, the switch is turned into an on-state (state of high resistance).
Referring to FIG. 10(a), if the voltage continues to be applied across the two electrodes, the metal precipitated on the surface of the second electrode is oxidized to form metal ions, so that metal precipitated now disappears.
The programming operation from the on-state to the off-state in FIG. 10(d) is termed a “reset operation.”
In the resistance change element, exploiting the solid electrolyte, switching between the “on-state” and the “off-state” is by formation and extinguishment of a conduction path via the metal crosslinking. Since the resistance change element has a simplified structure, its fabrication process is simple, such that it is possible to reduce the device size to a nanometer-order.
To overcome the problem of deterioration of a reference cell, there is disclosed in Patent Literature 1a semiconductor memory device in which the state of the reference cell is corrected efficiently to prevent the reference cell from being deteriorated due to disturbance to maintain the reference cell at highly accurate value. On the other hand, there is disclosed in Patent Literature 2 a semiconductor device the logic operation of which is programmable and which may also operate as a non-volatile memory device.    [Patent Literature 1] JP Patent Kokai JP-A-2004-185745    [Patent Literature 2] JP Patent Kokai JP-A-2008-235704